The present invention relates to voltage converters generally and more specifically to parallel arranged AC-to-DC regenerative converters.
Power is typically provided to factories, homes and the like via a utility grid including utility lines. Utility lines are usually bundled into line sets, each set including three utility lines, the lines together providing three phase AC utility voltages at relatively high utility amplitudes and low frequencies. In most cases, utility voltage amplitudes and frequencies are not suitable for driving electrical equipment and machinery and the amplitudes and frequencies must be modified prior to being used.
The most common way to modify utility voltage characteristics is to use an AC-to-DC rectifier for converting the three phase utility voltages to a DC voltage across positive and negative rails of a DC bus. A DC bus capacitor is provided across the rails which charges as current is provided to the rails. A DC-to-AC inverter is used to convert the DC voltage to three phase AC voltages on three feed lines which are linked to electrical equipment and machinery. The inverter can be regulated to control both frequency and amplitude of the resulting AC voltages on the feed lines. Machinery linked to the feed lines draws current from the DC bus.
In addition to drawing current from the DC bus, some machinery can operate in a reverse mode to provide current back through the feed lines to the DC bus. For example, while a motor draws current from the DC bus during motoring, the motor can operate as a generator during a braking process to provide current back to the DC bus. In essence, the inverter acts as a rectifier during braking tending, like the rectifier connected to the utility grid, to charge the bus capacitor.
When current is returned to the DC bus the total bus current can reach a level which will destroy the bus capacitor if the current on the bus is not reduced. One way to reduce bus current is to provide a switch in series with a braking resistor in parallel with the bus capacitor. If capacitor charge exceeds a threshold level, the switch is closed so that the braking resistor dissipates braking energy. Unfortunately, this way to reduce bus current is inefficient as the resistor effectively wastes energy returned to the DC bus.
A more efficient way to reduce bus current is to use a controllable regenerative, or switch-mode, rectifier. As the name implies, a regenerative rectifier can be controlled such that it operates in a regenerative or reverse mode to provide excessive bus current back to the utility lines. To this end, a regenerative rectifier includes at least six switches arranged to form three parallel legs between the positive and negative DC buses, each leg including two series connected switches. Switches linked to the positive DC bus will be referred to herein as upper switches and switches linked to the negative DC bus will be referred to herein as lower switches. Six separate diodes are arranged in inverse parallel relationship with the switches, a separate diode connected to each switch. Each of the three utility lines is connected to a rectifier input node between an associated pair of series switches via an input reactor (e.g. an inductor). Thus, the voltage difference between a supply line voltage and a rectifier input node voltage is impressed across a reactor therebetween.
In operation, to provide current to the DC bus, rectifier switches are alternately turned on and off such that a series of high frequency voltage pulses are generated at an associated rectifier input nodes. The RMS value of the resulting high frequency voltage pulses results in a low frequency alternating voltage at the input node. By controlling the high frequency pulses, the desired low frequency alternating voltage can be regulated. During motoring the switches are controlled to generate input voltages at the input nodes which lag the utility voltages and have a slightly smaller amplitude. Where the rectifier input voltages are slightly less than corresponding utility voltages, currents pass through the input reactors from the utility lines to the rectifier thereby providing currents to charge the bus capacitor.
To reduce DC bus voltage and return current to the utility lines, rectifier switches are controlled to generate rectifier input voltages which lead the utility voltages and have a slightly greater amplitude. Where the rectifier voltages are slightly greater than corresponding utility voltages, currents pass through the input reactors from the rectifier to the utility lines thereby sinking current from the DC bus and "regenerating" the current back to the utility lines.
To regulate a regenerative rectifier a controller is provided. The controller receives a DC bus voltage command signal and uses the voltage command signal to generate control signals to turn rectifier switches on and off in a sequence which will generate desired low frequency alternating voltages at the rectifier inputs. To this end, a controller typically includes, among other things, a voltage regulator, a current regulator, several voltage and current sensors, a carrier signal generator and a pulse width modulating (PWM) modulator. A voltage sensor is linked to the DC bus to sense the DC bus voltage and provide a DC feedback signal to the voltage regulator. The voltage regulator also receives a DC voltage command signal and compares the DC feedback and command signals to generate a current command signal.
A current sensor is linked to the utility supply lines and provides line current feedback signals to the current regulator. The current regulator also receives the current command signal and cooperates with several other controller components to compare the current feedback and current command signals to generate three modulating signals, one modulating signal for each of the three parallel rectifier legs. The carrier generator generates a triangle carrier signal having a frequency which is much higher than the modulating signal frequency.
The modulator receives and compares each modulating signal to the carrier signal. When a modulating signal is greater than the carrier signal, the modulator turns on a corresponding upper switch and turns off a corresponding lower switch. Similarly, when a modulating signal is less than the carrier signal, the modulator turns off a corresponding upper switch and turns on a corresponding lower switch. When an upper switch is on a corresponding utility line is linked to the positive DC bus and when a lower switch is on a corresponding utility line is linked to the negative DC bus. Thus, the modulator generates high frequency voltage pulses at the rectifier inputs, the average values of which can be regulated by changing the modulating signals.
The modulator generates modulating signals which have waveforms calculated to draw line currents i.sub.a, i.sub.b and i.sub.c from the utility lines such that: EQU i.sub.a +i.sub.b +i.sub.c =0 Eq. 1
The amount of current which can be passed by a rectifier switch is limited to a maximum level above which additional current will destroy the switch. For this reason, the maximum current which a rectifier can pass to the DC bus and a corresponding maximum DC bus voltage are also limited. Unfortunately, many industrial machines require much higher currents than a single rectifier can provide.
To increase the maximum current which can be provided via a DC bus, two or more regenerative rectifiers and corresponding controllers can be configured in a parallel relationship wherein each rectifier is linked between the three utility supply lines and a common DC bus. Bus capacitor size is increased to accommodate higher bus current levels. Each rectifier is capable of drawing current from the utility lines and providing the current to the DC bus, the combined rectifier currents capable of charging the bus capacitor to a voltage level and supply current which is approximately linearly related to the number of parallel rectifiers. For example, two rectifiers can supply current which is approximately twice as high as the current achievable using a single rectifier.
While parallel configured rectifier/controller systems can increase maximum bus current levels, they can adversely effect rectifier operation. An example of how a parallel configured rectifier/controller system can adversely effect rectifier operation is instructive. Assume that the parallel system includes only first and second rectifiers and associated controllers, each rectifier linking the utility lines to the DC bus. Rectifier and controller components corresponding to each of the utility lines operate in an identical fashion and therefore, to simplify this explanation, operation will only be described with respect to components corresponding to a first of the three lines. The first supply line is linked to a first rectifier input node (on the first rectifier) between a series connected upper/lower switch pair via a first reactor and is linked to a second rectifier input node (on the second rectifier) via a second reactor.
During operation, ideally, each of the upper switches is turned on and turned off at the same time and each of the lower switches is turned on and turned off at the same time. Where ideal operation occurs, identical voltages are generated across the first and second reactors and therefore each draws an identical amount of current from the first supply line.
In reality, however, while modulating signals for each of the first and second controller modulators and carrier signals provided by each controller carrier generator might be similar, they are almost never identical so that switches in the first and second rectifiers are almost never in sink. Where the first upper switch is on and the second upper switch in off, the DC bus voltage is impressed across the first and second inductors and causes a current spike through the first and second inductors. These current spikes often rise to several times the rectifier current ratings, prevent normal rectifier operation and can, over time, damage or even destroy rectifier components.
One solution to minimize current spikes due to asymmetrical switching sequences is to synchronize carrier signals and synchronize modulating signals for each parallel converter system. To this end, some in the industry have provided a single carrier signal generator which provides an identical carrier signal to each controller modulator. In addition, these solutions typically provide an identical current command signal to each controller current regulator so that modulating signals for each modulator are essentially identical.
While identical carrier signals and identical modulating signals reduce current spikes, independent rectifier current control and both software timing and hardware offset differences amongst the rectifiers and controllers render it almost impossible to generate symmetrical switching sequences. Thus, even where carrier signals are identical and modulating signals are identical, at least some current spikes still occur and adversely effect rectifier operation.
For this reason, it would be advantageous to have an apparatus which could eliminate current spikes through input reactors caused by asymmetrical switching sequences in parallel configured AC-to-DC regenerative converters.